"Netflix showed some increased latency, internal alarms went off but hasn't had a service outage." 
"Netflix is deployed in three zones, sized to lose one and keep going. Cheaper than cost of being down." 
For us, we are just now staffing up to the level where we can make the changes necessary to do the same thing.
Netflix are very very sensitive to this problem because it's much harder for them to sell against their biggest competitor (local cable) since they rely on the cable to deliver their service. If the service goes down, then the cable company can jump in and say, "You'll never lose the signal on our network" -- blatantly untrue, but it doesn't matter.
When you're disrupting a market, remember that what seem trivial is in fact hugely important when you're fighting huge well-established competition :)
I have not had a single service issue with them, ever. They do a better job at reliably providing me with TV shows than the cable company does. That seems to be where they're looking to position themselves, and the reputation for always being there is hard to regain if you lose it.
This is even more true if/when you load balance between zones and aren't just using them as hot backups. As another commenter pointed out, Netflix says they have three zones and only need two to operate.
And if they do mean three regions - can that cost of spanning various regions be quantified for different companies. The money spent vs money earned for Netflix may be very different compared to Quora and Reddit. At the same time, the data synchronization needs in between regions may also vastly differ for different type of companies and infrastructures thus leading to varying amount of cost to maintain a site on multiple regions.
1. See slides 32-35 of http://www.slideshare.net/adrianco/netflix-in-the-cloud-2011
2. "Deploy in three AZ with no extra instances - target autoscale 30-60% util. You have 50% headroom for load spikes. Lose an AZ -> 90% util."
Last 60 minutes comparison data: http://dl.dropbox.com/u/1898990/EC2-multiple-zones-60m.png
time in GMT.
A study we (Cedexis) did in January comparing multiple ec2 zones and other cloud providers: (pdf) http://dl.dropbox.com/u/1898990/76-marty-kagan.pdf